Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having a high-energy density and a high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbonaceous material as an anode active material, and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode and addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into a carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Such a lithium secondary battery must be basically stable in an operating voltage range of the battery and must have an ability to transfer ions at a sufficiently rapid rate.
The non-aqueous electrolyte is incorporated into the battery at the final step of fabrication of the lithium secondary battery. Here, in order to reduce a period of time taken to fabricate the battery and optimize the battery performance, it is necessary to ensure rapid and complete wetting of the electrodes by the electrolyte.
As the non-aqueous electrolyte for the lithium secondary battery, non-protic organic solvents such as ethylene carbonate (EC), diethyl carbonate (DEC) and 2-methyl tetrahydrofuran are largely used. Such an electrolyte is a polar solvent having a polarity to an extent that can effectively dissolve and dissociate electrolyte salts and at the same time, is a non-protic solvent having no active oxygen species. In addition, such an electrolyte often exhibits high viscosity and surface tension, due to extensive interaction of the electrolyte. Therefore, the non-aqueous electrolyte for the lithium secondary battery exhibits a low affinity for electrode materials containing a binder such as polytetrafluoroethylene, polyvinylidene fluoride and the like, and therefore results in a failure to achieve easy wetting of the electrode materials. Such a failure of easy wetting due to the low affinity, as will be illustrated hereinafter, is one of the primary causes which are responsible for an ineffective increase of the battery production time.
Meanwhile, miniaturization and very strict structure of the lithium secondary batteries have been recently required with increased demands thereof due to high preference toward small size devices such as mobile phones, notebook computers and MP3 players. Further, adoption of high-energy density batteries has led to increased electrode loading and thickness upon fabrication of the battery. However, due to a failure in deep penetration of the electrolyte having hydrophilic properties into the electrodes having hydrophobic properties, the capacity of the battery is lowered, which in turn reduces rate properties and cycle properties of the battery.
Therefore, conventional arts have been attempted to solve such problems by using specific process techniques such as addition of a high-temperate aging process or application of vacuum or pressure so as to promote wetting of the electrolyte on the battery. However, such methods suffer from a burden of extra expense for additional processes and a prolonged production period of time.
To this end, there is a strong need in the art for the development of a technology which is capable of reducing a battery fabrication time and improving the battery performance by increasing the wettability of the elecrodes to the electrolyte during fabrication of the battery.